1. Field of the Invention
The present invention relates to a plasma ion mass analyzing apparatus for identifying and measuring a minute amount of an element contained in a sample.
2. Description of the Related Art
A conventional structure will be described with reference to FIG. 3 in which a reference numeral 1 denotes a plasma generating device, and numeral 2 denotes a plasma generated by the plasma generating device 1. For example, the plasma generating device 1 may be an induction coupling plasma generating device disclosed in "Basic and Application of ICP Light Emission Analysis" (by Haraguchi, Kohdansha Scientific) or a microwave induction plasma generating device disclosed in Japanese Patent Application Laid-Open No. Hei 1-309300. A sample (not shown) to be analyzed is ionized by being introduced into the plasma 2 maintained by the plasma generating device 1. Numeral 3 denotes a sampling cone, numeral 4 denotes a skimmer cones and numeral 5 denotes a vacuum pump. The sampling cone 3 has at its tip end of conical shape an opening having a diameter of 0.8 to 1.2 mm. The skimmer cone 4 has at its tip end of conical shape an opening of 0.3 to 0.6 mm. A sampling interface is composed of the sampling cone 3 and the skimmer cone 4. When conducting an analysis, a line between the sampling cone 3 and the skimmer cone 4 is evacuated down to about 1 Torr by the vacuum pump 5 (generally, a rotary pump). Reference numeral 6 denotes a vacuum container, numeral 7 denotes an ion lens, numeral 8 denotes a mass analyzer, numeral 9 denotes a detector, and numeral 12 denotes a data processor. The interior of the vacuum container 6 is evacuated by other vacuum pumps 5. In a chamber where the ion lens 7 is disposed, the pressure is maintained at about 10.sup.-4 Torr, and in a chamber where the detector 9 is disposed, the pressure is maintained at 10.sup.-6 Torr. These vacuum pumps 5 may be generally turbo molecular pumps or oil diffusion pumps.
The sample which has been ionized by the plasma 2 is inputted into the ion lens 7 through the openings of the sampling cone 3 and the skimmer cone 4 together with rays of light of the plasma 2. The ion lens 7 serves to converge the ions and to introduce only ions from the sample. The mass analyzer 8 serves to introduce a predetermined mass out of the incident ions thereinto. For example, this analyzer 8 may be a quadrupole mass analyzer. The detector 9 serves to detect ions which have passed through the mass analyzer 8 and to feed an electric signal to the data processor 12. For example, this may be a Channeltron made by Galileo Co. The data processor calculates the mass of the ions from the setup of the mass analyzer 8 when the ions are detected by the detector 9 and specifies the kind of ions to thereby calculate a concentration of the ions specified by the detected intensity of the detector 9, i.e., the impurity contained in the sample.
The ion lens 7 will now be described with reference to FIG. 2 which shows a schematic view of the ion lens. In FIG. 2, reference numeral 13 denotes a sampling interface axis, characters 14a, 14b and 14c denote electrodes of a converging lens 14, characters 15a and 15b denote deflector electrodes of a deflector 15, numeral 16 denotes an aperture, and numeral 17 denotes an axis of the mass analyzer. The ion lens 7 is composed of the electrodes 14a, 14b and 14c of the converging lens 14, the electrodes 15a and 15b of the defector 15 and the aperture 16. The sampling interface axis 13 passes through the opening of the sampling cone 3 and the opening of the skimmer cone 4 and a beam of ions which has passed through the opening of the skimmer cone 4 enters the ion lens along the sampling interface axis 13. The converging lens 14 is composed of the electrodes 14a, 14b and 14c each of which is formed into a plate-like shape having an opening around the sampling interface axis 13. When a suitable voltage is applied to each of the electrodes 14a, 14b and 14c of the converging lens 14, the beam of ions is converged. Such a converging lens 14 is called an Einzel lens.
The deflector 15 is provided for displacing, in translation, the axis of the ion beam which has been inputted along the sampling interface axis 13. Namely, the ion beam that has been inputted into one electrode 15a of the deflector 15 is deflected by a predetermined angle, and the deflected ion beam is then deflected in the opposite direction by the same predetermined angle by the other electrode 15b of the deflector 15. The mass analyzer axis 17 corresponds to an optical axis of an incident window into which the species selected by the mass analyzer 8 is to be inputted and is positioned in parallel with the sampling interface axis 13 at an interval of about 10 mm. Thus, the axis of the ion beam (optical axis) is deflected from the sampling interface axis 13 to the mass analyzer axis 17. It should be noted that the ion beam is defected by the deflector 15, but light rays from the plasma 2 will not be inputted into the mass analyzer 8 or the detector 9 since light rays pass straight through the deflector 15 although the ray will pass through the openings of the sampling cone 3 and the skimmer cone 4 which are the sampling interface.
Only the ion species of the component to be detected, contained in the sample, passes through the mass analyzer 8 along the mass analyzer axis 17. The ion species which has passed through the mass analyzer 8 reaches the incident window of the detector 9, and the ion species of the desired component is detected by the detector 9. The detection signal from the detector 9 is inputted into the data processor 12 and is used for calculating the concentration of the desired component contained in the sample (i.e., a minute amount of the impurity).
Although the plasma ion mass analyzer apparatus has a high sensitivity, in the case where a high concentration sample is contained or a matrix component is contained at a high concentration in the sample, the high concentration components contaminates the interior of the apparatus and remains as a residue to adversely change the measurement value or stick to the interface portion, the ion lens, the mass analyzer and the detector within the apparatus to cause degradation in performance of the analyzer.
In case of the measurement of the sample, the occurrence of the above-described disadvantages is accelerated due to a period of time until the stabilization of the signal representative of the introduction of the sample into the plasma, and another period of time until the sample is eliminated from the plasma after the completion of the measurement, in addition to the period of time for the actual measurement. This is because the ion species of the sample component is introduced particularly into the detector of the plasma ion mass analyzing apparatus.